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Syncope After Vaccination --- United States, January 2005--July 2007

Syncope (vasovagal reaction), or fainting, can be triggered by various stimuli, including medical procedures
(1--3). Syncope has been documented to occur after vaccination, most commonly among adolescents, and can result
in hospitalization for a medical evaluation or because of injury
(2,4). During 2005 and 2006, the Advisory Committee
on Immunization Practices (ACIP) recommended use of three newly licensed vaccines for
adolescents*: the quadrivalent human papillomavirus recombinant vaccine (HPV)
(Gardasil®, Merck & Co., Inc., Whitehouse Station, New Jersey)
in a 3-dose series, the quadrivalent meningococcal conjugate vaccine (MCV4)
(Menactra®, Sanofi Pasteur, Inc.,
Swiftwater, Pennsylvania) in a single dose, and the tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis vaccine
(Tdap) (Adacel®, Sanofi Pasteur;
Boostrix®, GlaxoSmithKline Biologicals, Research Triangle Park, North Carolina) in a
single dose. To describe trends in occurrence of postvaccination syncope, CDC and the Food and Drug Administration
(FDA) analyzed data from the Vaccine Adverse Event Reporting System (VAERS) for January 1, 2005--July 31, 2007,
and compared the results with VAERS reports received during January 1, 2002--December 31, 2004. The
findings indicated that, since 2005, reports to VAERS regarding postvaccination syncope have increased, primarily
among females aged 11--18 years, and rarely, subsequent serious injuries have occurred. To prevent syncope-related
injuries, vaccine providers should follow the ACIP recommendation to strongly consider
observing patients for 15 minutes after vaccination
(4).

VAERS, a passive surveillance system operated jointly by FDA and CDC, receives reports of vaccine adverse
events (VAEs) and is designed to generate, not test,
vaccine-safety hypotheses
(5). Detecting new or rare VAEs,
monitoring trends in known adverse events, and identifying risk factors for particular types of VAEs are the primary objectives
of VAERS (5). Reports included in this analysis were those received by VAERS during January 1, 2005--July 31,
2007, that had VAEs coded as "syncope" or "syncope
vasovagal," on the basis of coding terms from the
Medical Dictionary for Regulatory
Activities
(MedDRA®).§ Reports to VAERS typically involve multiple coding terms. Because
vasovagal reactions have a relatively rapid onset and syncope is less likely to occur in young children, only reports of persons
who had syncope onset after vaccination on the same date and were aged
>5 years at the time of vaccination were included
in the analysis. Persons with either unknown age or unknown date of syncope onset were
excluded.

The rate of reports for postvaccination syncope was calculated by dividing the total number of reports by the
net number of doses of vaccine distributed in the United States each year (CDC, unpublished data, 1991--2006).
Patient characteristics, including age, sex, and vaccines
received, were compiled. To assess trends, these variables were
compared with VAERS reports of syncope during January
1, 2002--December 31, 2004. Adverse events were defined as serious
if one or more of the following patient outcomes were indicated in the report: death,
life-threatening illness, hospitalization, prolonged hospitalization, or permanent disability. For each serious event, the narrative descriptions
of
VAEs and medical records were reviewed by CDC medical officers to validate the diagnosis of syncope, determine
the interval between vaccination and onset in minutes, and identify any syncope-related injuries.

Following are selected case reports of postvaccination syncope in adolescents.

Case 1. A girl aged 13 years fainted within 10 minutes of receiving HPV and MCV4 vaccinations. She fell
backward and hit her head on the carpeted floor of the clinic. The girl was admitted to the pediatric intensive-care unit
because of skull fractures and subarachnoid hemorrhage. When VAERS contacted her approximately 6 months after the injury,
she had recovered completely.

Case 2. A girl aged 16 years felt dizzy and had pallor within 5 minutes of receiving an HPV vaccination. While
being escorted back to an examination room, she fainted, but the physician caught her as she fell. She was observed for
30 minutes in the clinic and recovered completely.

A total of 463 reports of postvaccination syncope during January 1, 2005--July 31, 2007
(Figure), were identified among persons aged
>5 years, compared with 203 reports during 2002--2004. The rate of reports for
postvaccination syncope among persons aged
>5 years were as follows: 0.30 reports per million doses distributed in 2002, 0.35
per million doses distributed in 2003, 0.28 per million doses distributed in 2004, 0.31 per million doses distributed
in 2005, and 0.54 per million doses distributed in
2006.¶ Compared with reports received during 2002--2004,
those received during 2005--2007 were more likely to
involve females (61.1% versus 77.5%) or persons aged
11--18 years (47.3% versus 62.0%) (Table). In 292 (63.1%) of the 463 reports during 2005--2007, syncope was associated with
at least one of the following recently approved and recommended adolescent vaccines: MCV4, Tdap, and HPV.

Thirty-three (7.1%) of the 463 postvaccination syncope reports during 2005--2007 were coded as serious (Table);
the percentage was not substantially different from the corresponding 20 (9.9%) serious reports during the
earlier comparison period. After clinical review, seven of the
reports coded as serious were excluded because they were either
not compatible with the diagnosis of syncope (n = 4)
or did not meet the criteria of seriousness (n = 3); 26 reports of
serious adverse events were analyzed further. The 26 patients ranged in age from 11 to 84 years
(median: 18 years), and 20 (76.9%) were female. Similar to the findings for syncope reports overall, females aged 11--18 years accounted for
the largest number of serious syncope reports (n = 11 [42.3%]). Among the 23 patients for whom times of vaccination
and syncope onset were indicated, 12 (52.2%) occurred within 5 minutes of vaccination, and 16 (69.6%) occurred
within 15 minutes. Ten of the 26 serious reports indicated that secondary injuries
occurred after syncope, including head injuries (n = 9)
after syncope-related falls and a motor-vehicle incident
(n = 1) because the patient lost consciousness while driving. Seven (70.0%) of the 10 secondary injuries occurred within 15 minutes of vaccination.

Editorial Note:

During 2005--2007, ACIP decided to add several newly licensed adolescent vaccines to the
routine immunization schedule and the Vaccines for Children Program. After these vaccines were licensed and recommended
for use, the number of postvaccination syncope reports to VAERS increased, primarily among females aged 11--18
years. Although only 7% of the reports met the criteria for being classified as serious, potentially life-threatening
injuries after postvaccination syncope were described, and one fatality was documented, resulting from intracranial
hemorrhage caused by head trauma in a boy aged 15 years
(7). ACIP and the American Academy of Pediatrics have
published recommendations to prevent postvaccination syncope and related injuries
(Box) (4,8). These preventive strategies
apply to all ages and all types of vaccines. However, the observed increase in postvaccination syncope and secondary
injuries suggests that adherence to the 15-minute postvaccination observation period and its efficacy in preventing
syncope-related injuries should be evaluated systematically.

The findings in this report are subject to at least four limitations. First, because of underreporting and lack of
age-specific data on vaccine doses administered, the rates calculated from VAERS data do not represent the actual
incidence rates of postvaccination syncope. The rates might be underestimated in this report because the denominators used in
the analysis were calculated from vaccine doses distributed, not doses administered, and syncope reports were excluded
for children aged <5 years, the population that receives the majority of vaccine doses. Second, hypotheses generated
from VAERS need additional clinical and epidemiologic analysis
(5). Although this report indicates that vaccines
most commonly noted in VAERS syncope reports are universally recommended for adolescents, this age group also has
a
higher background rate of syncope than other age groups
(9). The predominance of female patients in syncope
reports could reflect an actual difference in the occurrence of syncope between the sexes
(9). However, this predominance also could be a result of
reporting bias; the currently licensed HPV was recommended in a 3-dose series for females only,
and MCV4 and Tdap were each recommended for single-dose use in both sexes. Third, MedDRA coding terms might
not accurately reflect the diagnosis of syncope. The number of postvaccination syncope reports might be
either underestimated because certain syncope episodes might also be categorized as seizures or convulsions
(2) or overestimated because certain near-syncope or nonsyncope reports might be misclassified as syncope. Finally,
clinical details of nonserious reports were not reviewed; for example, although current recommendations suggest a
15-minute postvaccination observation period, data regarding distribution of minutes of time lapsed from vaccination to
syncope were not reviewed for nonserious reports.

All providers administering vaccinations should be aware of the potential for syncope after vaccination and should
take appropriate measures to prevent potential injuries. If syncope develops, patients should be observed until
symptoms resolve. In accordance with ACIP recommendations, providers should strongly consider observing patients for
15 minutes after they are vaccinated (4).

 Reports to VAERS can be made by anyone, including health-care providers, health departments, vaccine manufacturers, and members of the public. Any
clinically significant adverse events after vaccination can be reported; no confirmed causal relationship to vaccination is required. Limited mandated reporting exists for
health-care providers; however, vaccine manufacturers are required to report all adverse events that have been reported to them. Typically, such reports to
manufacturers originate from health-care providers. Published studies indicate that underreporting to VAERS varies but that events judged by the reporter to be serious are
more likely to be reported (6).

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